This invention relates to a hydraulic tensioner having a spring and check valve mounted within the piston. More particularly, this invention relates to a hydraulic tensioner in which the low pressure chamber, or reservoir, is located within the piston and the high pressure chamber is formed between the check valve located at the open end of the piston and the bottom of the bore in the housing. By locating the high pressure chamber outside of the piston, the size of the high pressure chamber is minimized, which allows a quick purge of air from the high pressure chamber upon start-up. Moreover, the location of the high and low pressure chambers in the present invention provides a hydraulic tensioner with a reduced overall package size.
A tensioning device, such as a hydraulic tensioner, is used as a control device for a power transmission chain as a chain travels between a plurality of sprockets. As a chain transmits power from a driving sprocket to a driven sprocket, one portion or strand of the chain between the sprockets will be tight while the other portion of the chain will be slack. In order to impart and maintain a certain degree of tension in the slack portion of the chain, a hydraulic tensioner provides a piston that presses against a tensioner arm or other chain guiding mechanism.
Prevention of excess slack in the chain is particularly important in the case of a chain driven camshaft in an internal combustion engine in that a chain without sufficient tension can skip a tooth or otherwise throw off the camshaft timing, possibly causing damage or rendering the engine inoperative. However, in the harsh environment of an internal combustion engine, various factors can cause fluctuations in the chain tension.
For instance, wide variations in temperature and thermal expansion coefficients among the various parts of the engine can cause the chain tension to vary between excessively high or low levels. During prolonged use, wear to the components of the power transmission system can cause a decrease in chain tension. In addition, camshaft and crankshaft induced torsional vibrations cause considerable variations in chain tension. Reverse rotation of an engine, occurring for example in stopping or in failed attempts at starting, can also cause fluctuations in chain tension. For these reasons, a mechanism such as a hydraulic tensioner is desired to ensure the necessary tension on the slack side of the chain.
Hydraulic tensioners are a common method of maintaining proper chain tension. In general, these devices employ a lever arm that pushes against the chain on the slack side of the chain. This lever arm must push toward the chain, tightening the chain when the chain is slack, and must provide resistive force when the chain tightens.
Typically, a hydraulic tensioner includes a piston in the form of a hollow cylinder. The piston slides within a bore in the housing and is biased outward from the housing in the direction of the chain by a spring. The interior of the piston forms a high pressure fluid chamber with the bore or opening in the housing. The high pressure chamber is connected through a one way check valve to a low pressure chamber or reservoir, which provides or is connected to an exterior source of hydraulic fluid. .
Upon start-up, the force of the spring on the piston causes the piston to move outward, which creates a low pressure condition in the high pressure fluid chamber, or pressure differential across the inlet check valve. Accordingly, the inlet check valve opens and permits the flow of fluid from the reservoir, or low pressure chamber, into the high pressure chamber. When the high pressure chamber is sufficiently filled with fluid, the force on the chain that moves the piston inward will be balanced by the outward force from the spring and the resistance force of the fluid in the chamber. The force of the chain against the fluid in the chamber also causes the check valve to close, which prevents further addition of fluid to the chamber.
The present invention achieves a reduction in the overall size of the tensioner unit by mounting the check valve in the end of the piston itself and locating the high pressure chamber between the check valve and the bore that receives the piston. The high pressure chamber is therefore located outside the piston, while the hollow central portion of the piston forms the low pressure chamber. Furthermore, the tensioner is able to purge air quickly because of the small volume of the high pressure chamber. As a result of the ability to purge air quickly, the use of a vent mechanism can be avoided, which eliminates the additional fluid leak path provided by the vent. various types of hydraulic tensioners are described in Suzuki et al., U.S. Pat. No. 5,352,159, Goppett et al., U.S. Pat. No. 4,792,322, and Sosson U.S. Pat. No. 4,850,941. The hydraulic tensioner of Sosson U.S. Pat. No. 4,850,941, has a check valve mounted in the piston, providing a relatively small high pressure chamber. The high pressure chamber is defined by part of the cavity formed in the housing and the piston. The tensioner does not have a spring between the body and the piston or a means for permitting discharge of air from the chamber.
The hydraulic tensioner of U.S. Pat. No. 4,772,251 has a check valve mounted in the base of the piston. The main spring is located outside of the piston. A separate clip is used to keep the piston and body from separating.